People who obtain a US Immigrant Visa are required to pay a "USCIS Immigrant Fee", for Green Card processing. The official instructions recommend to pay this fee before entry to the US, so you receive the GC in minimum time. They also mention it's possible to pay it after entering the US.
Could someone with a valid visa, who hasn't yet payed this fee, be denied to board on the flight to US, when going through security airport?
This got answered on travel.SE before it was put on hold there, so I'll copy the answer here for any expats who need the information:
Yes, they will be allowed entry, and they will become a permanent resident immediately upon entry as usual. They just won't be mailed the physical plastic green card. However, their immigrant visa, endorsed upon entry, serves as a green card for one year, so they will still be able to prove their permanent resident status without the plastic card in the short term.
